The present invention relates to satellite communications systems, which include communication between two moving satellites, between one moving satellite and a ground-based station (moving or stationary), between a space-based vehicle and an airborne (but not space-based) vehicle, or any combination of the above.
In a satellite communications system, one satellite vehicle (SV1) transmits and receives data from another satellite vehicle (SV2). Even though one or both vehicles may be moving, if the relative distance between the two vehicles is constant, the carrier frequency on which data is transmitted from one vehicle to another is the same carrier frequency received by the receiving vehicle. In the more typical case, the relative motion between the two vehicles in communication is not constant, and Doppler effects come into play. As long as the relative motion between the transmitter and receiver is within the design constraints of those devices, classical techniques, such as those described in Spiker, James J., Digital Communications by Satellite, Prentice-Hall, Ch. 12, 1995, may be employed to compensate for the carrier frequency shift due to the relative motion between the two vehicles. For example, a frequency-locked loop or a phase-locked loop technique may be employed to track Doppler frequency shifts and maintain communication between the two vehicles. However, these classical tracking techniques often involve broadening the bandwidth of the frequency tracking device and/or the bandwidth of the receiver""s noise limiting front-end filter, which tends to reduce the signal-to-noise ratio of the received signal. When the relative motion between the transmitter and the receiver is unacceptably high, the signal-to-noise ratio can become unacceptably low, rendering known compensation techniques insufficient.
A technique is thus needed which allows satellite vehicles to communicate with one another when the relative motion and the change in relative motion between two vehicles is high, while maintaining an acceptable signal-to-noise ratio of the received signal.